An example of the original code for an interview

input_multiform_seller.dart

//Screen to manage MAP and Description forms
import 'dart:collection';
import 'package:flutter/material.dart';

import 'package:flutter_map_live1/ui/widgets/seller_input/input_map_seller.dart';
import 'package:flutter_map_live1/ui/widgets/seller_input/input_description_seller.dart';
import 'package:flutter_map_live1/ui/widgets/seller_input/input_upload_seller.dart';

class StepperPage extends StatefulWidget {
  StepperPage({Key? key, required this.title}) : super (key: key);

  final String title;

  @override
  _StepperPageState createState() => _StepperPageState();
}

class _StepperPageState extends State<StepperPage> {
  int currentStep = 0;
  GlobalKey<FormState> _formKey = new GlobalKey<FormState>();

  static final pointController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    var mapData = HashMap<String, String>();
    mapData["seller_point"] = SellerMapInputState.pointController.text;
    mapData["seller_name"] = SellerDescriptionInputState.nameController.text;
    mapData["seller_type"] = SellerDescriptionInputState.typeController.text;
    mapData["seller_transaction"] = SellerDescriptionInputState.transactionController.text;

    List<Step> steps = [
      Step(
        title: Text('LatLng'),
        content: SellerMapInput(),
        state: currentStep == 0 ? StepState.editing : StepState.indexed,
        isActive: true,
      ),
      Step(
        title: Text('Description'),
        content: SellerDescriptionInput(),
        state: currentStep == 1 ? StepState.editing : StepState.indexed,
        isActive: true,
      ),
      Step(
        title: Text('Upload'),
        content: Upload(mapData),
        state: StepState.complete,
        isActive: true,
      )
    ];

    return Scaffold(
      body: Column(
        children: <Widget>[
          SizedBox(
            height: 20.0,
          ),
          Expanded(
            child: Stepper(
                controlsBuilder: (BuildContext context,
                    {VoidCallback? onStepContinue, VoidCallback? onStepCancel}) {
                      if (currentStep < steps.length - 1) {
                        return Row(
                          children: <Widget>[
                            TextButton(
                              onPressed: onStepContinue,
                              child: const Text('NEXT'),
                            ),
                            TextButton(
                              onPressed: onStepCancel,
                              child: const Text('CANCEL'),
                            ),
                          ],);
                      } else { return Row();}
                },
              currentStep: this.currentStep,
              steps: steps,
              type: StepperType.horizontal,
              onStepTapped: (step) {
                setState(() {
                  currentStep = step;
                });
              },
              onStepContinue: (){
                setState(() {
                  if (currentStep < steps.length - 1) {
                    if (currentStep == 0 && SellerMapInputState.formKey.currentState!.validate()){
                      currentStep = currentStep + 1;
                    } else if (currentStep == 1 && SellerDescriptionInputState.formKey.currentState!.validate()) {
                      currentStep = currentStep + 1;
                    } else {
                      currentStep = 0;
                    }
                  }
                });
              }
            ),
          ),
        ]
      ),
    );
  }
}